Whenever "functional illness" and ME are mentioned in the same sentence I like to quote Rachel Jenkins:
HYSYERIA RECONSIDERED
As we have seen, right from the early epidemics, the question of hysteria was considered by observers, who nonetheless decided that the evidence pointed to an organic aetiology. However since the McEvedy and Beard papers were published, the standard psychiatric teaching, and indeed often the standard medical teaching, has been that ME is primarily a functional illness, namely hysteria. Before reassessing this viewpoint, it is important to establish precisely what we mean by hysteria. The term has been used in many different ways...……
She goes on to consider the question in some detail in the Introduction of Post-viral Fatigue Syndrome (Myalgic Encephalomyelitis) 1991 eds Rachel Jenkins and James Mowbray. She should have known what was taught. She was, or had been, a lecturer at the Institute of Psychiatry.
It looks as though those who consider the condition "functional" should be questioned as to which precise use of "hysteria" they intend.